Resistance for electrical apparatus.



No. 806,758. PATENTED DEC. 5, 1905.

O. P. STEINMETZ.

RESISTANCE FOR ELECTRICAL APPARATUS.

APPLICATION FILED JAN. 21, 1901.

' Witnesses. Inventor.

M CharlesPSCeinmetz,

AC'Ey.

UNITED STATES PATENT OFFICE.

CHARLES P. STEINMETZ, OF SCHENECTADY, NET/V YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NET/V YORK.

RESISTANCE FOR ELECTRICAL APPARATUS.

Specification of Letters Patent.

Patented Dec. 5, 1905.

Application file January 21, 1901. Serial No. 44,067.

T0 at whom it may concern:

Be it known that I, CnARLEs I. STEIN- METZ, a citizen of the United States, residing at Schenectady, county of Schenectady, State of New York, have invented certain new and useful Improvements in Resistances for Electrical Apparatus, of which the following is a specification.

My invention relates to a resistance for use in connection with electrical apparatus which is designed to automatically and gradually cut itself out of the circuit after a cer tain quantity of current has flowed through it. Such a resistance is of special advantage in starting electric motors and takes the place of the hand-operated rheostats now in common use for that purpose.

The self-reducing resistance which I have invented consists of a substance which has a high initial resistance, but becomes more and more conductive as its temperature rises by reason of a continual flow of current until its resistance finally becomes so small asto enable it to pass the full current to the motor with a practicallynegligible ohmic drop through the resistance, whereas at starting the full-line voltage may be required at the terminals of the resistance to force the same current through, this action being identical with that of the usual hand-operated rheostats, as stated above.

It has long been known that certain substances which are capable of conducting electric currents at ordinary temperatures have negative temperature coefficientsthat is, the resistance which they offer to the flow of an electric current decreases with an increase in temperature. As an example of a substance of this class I may mention carbon.

More recently it has been discovered that some of the rare earths which are practically non conductors at ordinary temperatures have their resistance so much reduced at high temperatures that they become fairly good conductors and may be kept at incandescence or even melted when connected to a source of electric current the voltage of which is insufficient to cause any substantial flow of current therethrough when cold.

I have discovered that certain chemical compounds or mixtures combine in a way the characteristics of both of the classes of substances above referred to, being comparatively good conductors at ordinary temperatures and having also a wide range of resistance with increase in temperature, so that they may be used as automatic starting resistances in electric circuits, becoming gradually more conductive as the current flows therethrough until finally they oiIer but little resistance to the current-flow. Among the various substances which are conductive at ordinary temperatures and become more conductive as the temperature increases I find that the tetroxid of iron or magnetite is especially suitable for such a purpose, or I may use a mixture of magnetite and chromite of iron, the latter having a higher initial resistance than the former, but being equally conductive at high temperatures. These substances are chemically stable and structurally solid even at a red heat, while the resistance of magnetite is about one hundred times less at red heat than at ordinary temperatures. By mixing with it the chromite of iron the initial resistance can be made even higher.

In the accompanying drawings, Figure 1 shows a series motor and a shunt-motor provided with my self-reducing starting resistance. Fig. 2 is a diagram showing the illvention applied to an induction-motor. Fig. 3 is a sectional end view of a portion of the squirrel-cage armature of an induction-motor, showing another mode of applying my invention; and Fig. 4 is a sectional side elevation of the same.

The series motor A and the shunt-motor B are shown connected across the positive and negative mains C C, and the circuit is controlled by a double-pole switch D. The self-reducing resistance is shown at E, being simply a stick of magnetite or other material having the characteristics above specified suitably mounted in series with the armature of the motor. When the switch is first closed, the stick E being cold offers a high resistance to the passage of the current; but as the current heats it up it becomes more conductive until finally when at a red heat it ofi ers practically no obstacle to the passage of the current. The length of time required to reach this point depends upon the size of the stick E and the voltage to which it is subjected.

Fig. 2 shows in diagram the connections of the armature of an induction-motor, the ar mature-coils being in three sets F F F, all connected at one end to a short-circuiting ring G and each at the other end to a segment of a short-circuiting ring H. The gaps in the ring II are bridged by resistances E, and the effect of this construction is to check at first any rush of induced current in the armature, but gradually allow it to increase as the speed of the motor increases and the current heats up the resistance.

Another mode of applying the invention to induction-motors is shown in Figs. 3 and 4. The armature-bars I of a squirrel-cage armature are usually secured at each end to a short-circuiting ring K. I place between one or more of the bars and the ring one or more washers L, of magnetite or the like, preferably separated from the bar and ring and from each other by washers M of copper or other good conducting material. The fastening-bolt N is insulated from the wash ers and the ring to compel the current to pass through the washers. hen the motor starts, the magnetite washers are cold and offer considerable resistance to the otherwise heavy rush of secondary current; but as the continual flow of current heats the washers they become more conductive and finally permit current to pass freely. Then the motor stops, the heat of the washers is quickly conducted away and dissipated by the ring K, which has considerable radiating surface.

What I claim as new, and desire to secure by Letters Patent of the United States, is

1. A self-reducing starting resistance for electric motors, consisting of an oxid of iron.

2. A self-reducing starting resistance for electric motors, consisting of a tetroxid of iron.

3. In combination, an electric motor, a source of current therefor, and a self-reducing starting resistance consisting of a substance connected in circuit with one of the windings of said motor, which at starting permits the flow of normal starting-current only through said winding and when heated by said flow becomes a practically-negligible resistance.

4. In combination, an electric motor, a source of current therefor, and a self-reducing starting resistance consisting of a sub stance connected in circuit with one of the windings of said motor, which when cold pro' duces a large ohmic drop when traversed by the normal current of said winding and which when heated by said current produces a practically-negligible drop.

5. The combination with the induced windings of an induction-motor, of a self-reducing resistance therefor.

6. In an induction-motor, the combination with a short-circuiting ring, of a piece of mag netite or the like in circuit therewith.

7. In an induction-motor, the combination with the armature-winding and a short-circuiting ring, of magnetite or the like between said winding and ring.

8. In an induction-motor, the combination with an armature-bar and a short-circuiting ring, of a piece of a self-reducing resistance material between said bar and ring.

9. In an induction-motor, the combination with an armature-bar and a short-circuiting ring, of alternate washers of a self-reducing resistance material and good conducting ma terial placed between said bar and ring.

10. The combination with an electric motor, of a starting resistance therefor of marked negative temperature coefficient adapted to permit the How of normal starting-current only to said motor and when heated to produce a practically-negligible resistance.

11. In combination, an electric motor, a source of current therefor, and a self-reducing starting resistance consisting of a stable and structurally-solid substance connected in circuit with one of the windings of said motor, which at starting permits the flow of normal starting-current only through said winding and when heated by said flow becomes a practically-negligible resistance.

In witness whereof I have hereunto set my hand this 18th day of January, 1901.

CHARLES P. STEINMETZ.

I/Vitnesses:

BENJAMIN B. HULL, EDWARD WILLIAMS, Jr. 

